Chinese authorities found no formamide in hundreds of baby diaper samples tested in a nationwide investigation prompted by public concern over the substance, the country's market regulator said on Thursday.

The investigation covered 168 domestic manufacturers, including contract producers, and tested 368 batches of finished diapers across 180 brands, the State Administration for Market Regulation said.

Tests conducted by institutions, including the Chinese Center for Disease Control and Prevention, detected no formamide, the regulator said. Investigators also found no evidence that the substance had been added during production or was present in the raw materials used.

The manufacturers inspected all met hygiene requirements for the production of disinfection products, it added.

The investigation was conducted jointly by the market regulator, the Ministry of Industry and Information Technology, the National Health Commission and several other authorities.

Investigators found flaws in the work of several testing bodies that had claimed to detect formamide in baby diapers, the regulator said.

These included unclear criteria for identifying the substance, failures to test blank and duplicate samples, insufficiently rigorous testing procedures and assessment criteria, and the deletion of raw data.

Those tests could not support a conclusion that formamide had been detected, the regulator said.